This invention relates to the calibration of currency validators. It is applicable both to banknote validators and coin validators, and to the initial calibration of the validators in the factory of a manufacturer and to the re-calibration of validators in the field.
It is well known that currency acceptors, or validators, require calibration to take into account small differences in the sensor responses to currency articles. One common technique for calibration (see for example GB-A-1 452 740) involves taking measurements of currency articles, storing data (for example upper and lower limits) associated with these measurements, and subsequently testing articles by determining whether measurements of the articles are consistent with stored data.
This procedure permits very reliable operation, but the calibration procedure can be very time consuming. Each apparatus has to measure a statistically significant number of articles of the denominations, or classes, which the apparatus is subsequently operable to recognise. Techniques for reducing the amount of time and effort required during calibration have been proposed. See for example GB-A-2 199 978.
It is also known for validators to have an automatic re-calibration function, some times known as xe2x80x9cself-tuningxe2x80x9d, whereby acceptance criteria are regularly updated on the basis of measurements performed during testing (see for example EP-A-0 155 126, GB-A-2 059 129 and U.S. Pat. No. 4,951,799). This technique is useful in that it can take account of changes in the characteristics of the individual apparatus.
Generally speaking, calibration techniques often require the validator to be placed in a special calibration mode, and involve controlled conditions in which the measured articles are of known classes. Accordingly, the measurements can be treated as reliable, although it is still necessary to take into account possible xe2x80x9cflyersxe2x80x9d, i.e. articles which, because of unusual circumstances, fail to be measured in the appropriate conditionsxe2x80x94see for example EP-A-0781439.
Self-tuning techniques, on the other hand, take advantage of the fact that the apparatus is already calibrated. Accordingly, the apparatus can use measurements of articles tested and found to belong to a certain class for the purposes of re-calibrating, which generally takes the form of adjusting the acceptance criteria for the particular class. However, a problem with this technique is that the classification may not be accurate, and therefore it is possible for re-calibration to result in a deterioration in reliability unless special measures are taken to prevent this.
It would be desirable to provide a technique for calibrating acceptors which can be carried out more quickly and more easily than the prior art techniques.
Aspects of the present invention are set out in the accompanying claims.
According to a further aspect of the invention, an uncalibrated mechanism is used to classify articles measured by the apparatus, using generic classification criteria which are common to this and other apparatuses. Calibration of the apparatus is then performed using both the measurements and the classifications.
This technique differs from conventional calibration techniques in that the apparatus is itself used to classify the articles measured in the calibration process. In normal calibration techniques, each inserted article was of a predetermined, known class, and the uncalibrated mechanism was not relied upon to classify the article. It has, however, been found that even an uncalibrated mechanism, when supplied with articles known to belong to a certain group of classes, can reliably allocate each article to the correct class. Accordingly, the articles used in calibration can be fed in any sequence, thus simplifying the calibration procedure. This technique also differs from self-tuning techniques, in which the classification is performed by a calibrated apparatus, and in which received articles are not known to belong to specific classes.
Preferably, each article is recognised by using data derived from correlations between different article measurements in a population of the respective class. Preferably, the articles are classified by normalising at least some of their measurements, using one or more other measurements as normalisation factors, so as to reduce acceptor-to-acceptor variations in the classification criteria.
According to a still further aspect of the invention, the measurements of articles derived during a calibration procedure are subject to an integrity check to determine whether they should be used for calibration purposes. Different types of integrity checks may be used. The first type of integrity check involves comparing different measurements of an article with each other. Preferably, the comparison operation involves determining whether the relationship between those measurements matches a correlation which has been found in populations of articles of the relevant class. If the relationships do not match this correlation, then the measurements of the article are deemed inappropriate for use in calibration.
Another integrity check involves comparing a first type of measurement of an article with corresponding measurements of other articles. This comparison stage also preferably involves determining whether the relationships between the measurements matches a statistical correlation found by evaluating populations of the relevant calibration classes. A similar operation can be performed on the other measurements of the respective articles. This integrity check is preferably repeated, each time using a different article for normalisation purposes. This allows articles with unrepresentative measurements to be distinguished from properly-measured articles.
The various aspects of the invention are particularly useful for enabling calibration to be carried out in a very quick and simple manner involving measurements of only a relatively small number of articles, and preferably only a single article of each of a relatively small number of known classes. This would normally result in a high risk of calibration being carried out incorrectly, but the integrity checks can rapidly detect whether any of the measured articles is unrepresentative, for example if it is a xe2x80x9cflyerxe2x80x9d, in which case the measurements from that article can be disregarded.
In a preferred embodiment, the calibration procedure involves measuring, in any sequence, a small number of articles belonging to respective different classes, for example one of each class, and providing an indication if any integrity check is failed, so that at least one of the articles can be re-measured. Preferably, the indication is capable of identifying an article which caused the integrity check failure so that if desired only this class of article needs to be re-measured.
The invention is applicable to various types of calibration procedures. For example, the article measurements can be used to set ranges used as acceptance criteria for recognising other articles belonging to the same classes as those used during calibration, similar to the procedure in GB-A-1 452 740. Alternatively, or additionally, the measurements can be used to derive acceptance criteria for different classes, similar to the techniques used in GB-A-2 199 978. A further possibility is to use the calibration data to adjust measurements made of articles before these measurements are checked against acceptance criteria.